1. Field of the Invention
The field of the invention includes the resistivity of materials, and, more specifically, methods and devices for measuring the volume resistivity of fluid materials, such as a powder. The volume resistivity of energetic materials may be measured using the device and methods of the present invention.
2. State of the Art
Volume resistivity is the ratio of a dc voltage drop per unit thickness to the amount of current per unit area passing through a material. Volume resistivity indicates how readily a material conducts electricity through the material. The volume resistivity may also be useful to examine physical characteristics of materials such as electrostatic decay and thermal characteristics, for example, heating. Petrochemical, grain, and semiconductor industries rely on knowledge of the volume resistivity of energetic materials to prevent product and personnel loss.
One example of a conventional device for measuring resistivity is described in U.S. Pat. No. 4,922,182 to Cox. The device is capacitively coupled to a material under test. The tested material may include two outer layers and a solid, conductive buried layer. Pickup and driven electrodes are capacitively coupled at different locations to the buried layer. A frequency may be generated which resonates the capacitively coupled pickup and driven electrodes with an inductor. The voltage output of this series resonant circuit may be measured, and the resistivity of the buried layer may be determined using Ohm's law.
The pickup and driven electrodes of this conventional device are coupled to capacitive plates integral to the conductive buried layer. However, a fluid material, one which flows and conforms to the shape of a container, such as a powdered or granular material, cannot have an integral capacitive plate. Therefore, the resistivity of a fluid material cannot be measured with this conventional device.
Energetic materials are materials that react chemically to release energy. Explosives, pyrotechnics and propellants are examples of energetic materials. An energetic material may be initiated to undergo rapid, self-propagating decomposition that results in the formation of more stable material, the production of heat, or the development of a sudden pressure effect through the action of heat on produced or adjacent gases. Energetic materials are especially vulnerable to initiation when subjected to an elevated temperature. Energetic materials may also be initiated by mechanical work through friction, impact, or electricity. Energetic materials may be used, for example, as explosives in demolition, welding, detonating, cutting charges and munition fillings, as propellants for guns and rockets, as gas generators or as pyrotechnics. Energetic materials are often used in fluid, i.e., powdered or granular, form.
Therefore, there is a need for a device for measuring volume resistivity of a fluid material. There is additional need for a device for measuring volume resistivity of a energetic material, particularly in a fluid form.